The ANBU is actually short for Ansatsu Senjutsu Tokushu Butai (literally Assassination Tactics Special Force), and is an organization of assassination squads that serves directly under the village’s leader. It is still unclear as to what requirements a shinobi must possess to be an ANBU operative.
The ANBU wear wood animal masks in order to distinguish themselves from normal shinobi and to conceal their identities, and also have standard uniforms consisting of black and gray armor, arm guards, a signature spiral tattoo on their shoulder (Men: left shoulder. Women: right shoulder) and a Ninjato strapped on their back. They know the layout of the human body in detail, and have developed several techniques to perform their duties, such as paralyzing the target.
ANBU are considered the elite, and they perform high-ranked and dangerous missions. An ANBU may retire their duty to return to the life as Chunin or Jonin, as in the case of Kakashi Hatake. Within the ANBU of Kirigakure, and presumably other ninja villages, there exists a sub-group called Hunter-nin (literally “Chasing Ninja”, English TV “Tracker Ninja”, given the specific duty of hunting down rogue ninja, and destroying their bodies, so their village’s secrets will not be revealed to foreign ninja.